Chandu,
Text box used to have a limit on the number of lines that it contained. CVI 6 and up include a text box with no set limit for the number of lines that it can contain. Basically the limit will depend on the memory available.
The best practice is to always check the return value of the function to set the text. You should get an out of memory error once you have reached the limit of what the text box can hold.
If you want to set a limit, it's up to your program to check the number of lines added so far and determining if there is space for more characters.
I hope this helps, let me know if you have any further questions on this.
Regards,
Juan Carlos
N.I.